I have XM, so I'm a bit biased
XM's technology is years ahead of Sirius - I have a Pioneer Inno that works with Napster as an mp3 player. I think it holds 50 hours of programming/music. You can record shows on XM & listen to them later & it's portable, so that you can walk around outside & listen to LIVE programming (as far as I know, Sirius has a unit that will let you record, but not listen to live programming as you walk around)
But if none of that matters to you, it boils down to this for men:
XM has Opie & Anthony, MLB & NHL
Sirius has Stern, NFL & NBA
Of course, once this merger goes thru, it'll all change & we're supposed to be able to pick & choose what we want (like tiers on cable TV)
Go here & poke around:
http://www.myradiostore.com/
(I make no money thru that link - I just use them & they are good people)